0
You can try rosy barbs and / or rosy (fathead) minnows. Both are schooling fish, and you probably shouldn't have more than four or five of the barbs in a 40-gal. I have seven in my 65.
Acclimate all new fish well. I've found that minnows don't do well if they're not properly acclimated.
Glofish danios in a variety of colors, or simple zebra and / or leopard danios are another option.
Stay away from goldfish. Your tank is too small and would be too crowded with them in there.
Dojos are cute, aren't they?
20 gal. high: planted; 5 white cloud minnows, 4 golden White Clouds, several RCS, 2 blue shrimp, 5 Amano shrimp, several snails; Azoo air. 65 gal: planted; 6 rosy barbs, 6 yellow glofish, 3 red glofish, 3 zebra danios, 5 white cloud minnows, 3 dojo loaches, 6 crimson spot rainbow fish, 12 large Amano Shrimp, several snails; AC110.